Heat exchanger perforated fins

ABSTRACT

A plate fin heat exchanger comprises a folded fin sheet comprising fins wherein the fin sheet comprises a plurality of perforations, such plurality of perforations are positioned on the fin sheet in parallel rows when such fin sheet is in an unfolded state, such parallel rows of perforations on the fin sheet comprise a first spacing between the parallel rows of perforations (S 1 ), a second spacing between sequential perforations within the parallel row of perforations (S 2 ), a third spacing (or offset) between the perforations in adjacent parallel rows of perforations (S 3 ), and a perforation diameter (D), wherein the ratio of the first spacing between the parallel rows of perforations to the perforation diameter (S 1 /D) is in the range of 0.75 to 2.0, and wherein the angle between the fins and the parallel rows of perforations is less than or equal to five degrees (≦5°).

BACKGROUND

Plate fin heat exchangers are generally used for exchanging heat betweenprocess streams for the purpose of heating, cooling, boiling,evaporating, or condensing the process streams. The process conditionsin these heat exchangers may involve single phase or two phase flow andheat transfer. While some plate fin exchangers contain only two streams,others contain multiple streams in multiple sets of plate fin passages.Individual streams may be fed into and withdrawn from the heat exchangerusing nozzles and headers. Each stream flows into specific plate-finpassages allocated within the bank of adjacent plate-fin passages. Theindividual plate-fin passages are contained between pairs of partingsheets, which are spaced apart by the fins and the plate-fin passagesare enclosed on the outer periphery by sidebars and endbars so they canbe isolated from each other and can contain the fluids of interest. Whenstreams at different temperatures flow in the plate-fin passages thatare adjacent to each other, they exchange heat through the partingsheets which are referred to as primary heat transfer surfaces as wellas the fin legs that separate them, which are referred to as secondaryheat transfer surfaces.

Plate fin exchangers may be formed by using many different types of finssuch as plain, perforated, serrated and wavy. One embodiment of thecurrent invention deals with perforated fins which have been employed inthe industry, but in an inefficient manner. The plate fin heatexchangers having perforated fins, according to the present invention,have particular application in cryogenic processes such as airseparation, although these plate fin heat exchangers may be used inother heat transfer processes.

When a stream or fluid enters a plate fin heat exchanger channel itexhibits high heat transfer coefficients due to the well-known entranceeffect. Post entrance effect, the stream or fluid will soon reach asteady state condition with a much lower heat transfer coefficient. Inparticular when the flow is characterized as being in a turbulent stateor in a transition state between laminar and turbulent states, laminarand viscous boundary layers are known to form adjacent to all thesurfaces that the fluid flows along. The overall effect is to lower theaverage heat transfer coefficients in such an exchanger. The lower heattransfer coefficient condition can be at least partially reversed byperiodically disturbing this boundary layer through a variety of meanssuch as, for example, introduction of perforations or serrations in thefins. Introduction of perforations or serrations in the fins willincrease the heat transfer performance, however, such introduction willalso increase pressure losses and, therefore, the geometry andarrangement of the perforations or serrations in the fins is criticalfor achieving improved performance. It is particularly important in thecase of perforated fins because while they disturb the flow leading toan increase in the local heat transfer coefficient proximate to theperforations, introduction of perforations in the fins also results in aloss of surface area from the original material which would otherwisehave been beneficial for the overall heat transfer from the heatexchanger. Also removal of metal, for example, in the form ofperforations can greatly reduce the strength of the remaining material.Thus, the problem of improving the performance of plate fin heatexchangers by using perforated fins is complicated and it isparticularly important to organize the geometry and arrangements ofusing such perforations to achieve improved performance.

Historically, publications concerning plate fin heat exchangers providedgeneral descriptions of the overall geometry and the elementary methodsfor the manufacture of plate fin heat exchangers. While thesepublications discuss the many constituent parts of plate fin heatexchangers, their relationship to one another, and how they areassembled and brazed together, the publications are brief in theirdescription of the perforated fins that may be utilized in such platefin heat exchangers. Even in cases where some nominal details aredisclosed, the publications simply fail to discuss any preferredgeometry and patterns to use.

For example, in “Aluminum Brazed Plate Fin Heat Exchangers for ProcessIndustries,” a chapter of Compact Heat Exchangers for the ProcessIndustries, edited by R. K. Shah, proceedings of the InternationalConference for the Process Industries, held at Cliff Lodge andConference Center, Snowbird, Utah, Jun. 22-27, 1997, by Shozo Hotta fromSumitomo Precision Products (SPP), a general description of plate finheat exchangers by SPP, a major supplier of such heat exchangers, isdisclosed. Specifically, FIG. 4 on page 181 of such reference providesphotographic evidence of the common fin types including perforated fins.As described and taught therein, the perforated fins are formed byfolding a sheet with regularly perforated small round apertures orperforations at some large angle relative to a major axis ofperforations on the flat sheet. No further details, however, arepresented.

This method of manufacture is very common in the industry to minimizethe overall cost. A few standard perforated sheet materials may be usedto produce a wide range of finished fins with varying dimensions. Thistype of method of manufacture of perforated fins, however, leads to anirregular arrangement of the perforations on the fins resulting in poorperformance of the perforated fins.

U.S. Pat. No. 6,834,515 B2, entitled “Plate Fin Exchangers with TexturedSurfaces,” to Sunder et al., also discloses various perforated fins. TheSunder patent teaches use of surface texture to enhance the performanceof other perforated fins. FIG. 2B of the Sunder patent illustratesexemplary fins with a row of perforations along the top and sides of thefins where the perforations are laterally aligned. Example 1 of theSunder patent states that the perforated fins have an open area of about10%. No other details, however, are provided regarding the perforations.

U.S. Pat. No. 5,603,376, entitled “Heat Exchanger for ElectronicsCabinet,” to Hendrix, describes a heat exchanger for passive heatexchange between a weather-tight, sealed electronics cabinet, and theoutside environment. FIG. 2 of the Hendrix patent shows heat generationside fins 21 with perforations 25 contained therein. The Hendrix patentteaches that fins 21 are formed by pleating or folding perforated sheetmaterial. The perforations are said to be perpendicular to the directionof the folds. FIG. 2 of the Hendrix patent illustrates that theperforations are a single row of perforations along the sides of fins21, however, no perforations are shown on the underside where thevalleys or crests of the waves would form. Further, the Hendrix patentprovides no teaching regarding the position of the perforations.

In “Three-dimensional numerical simulation on the laminar flow and heattransfer in four basic fins of plate-fin heat exchangers”, by Y. Zhu andY. Li, Journal of Heat Transfer, November 2008, vol. 130, 111801-1 to 8,a Computational Fluid Dynamics (CFD) based calculation is performedconcerning the performance of four samples (plain, perforated, stripoffset (which is another term for serrated) and wavy fins) is disclosed.The Zhu and Li paper lists many major publications on compact heatexchangers that have appeared since they were first introduced, and goeson to state that, “[t]o the best of the authors' knowledge, completethree-dimensional flow and heat transfer in the perforated fins havereceived scant attention in literature.”

Such statement is significant and appears to support and lead to theconclusion of Applicants, namely that what is known in the artconcerning perforated fins is sub-optimal.

As part of comparing the four types of fins, the authors of the Zhu andLi paper conducted CFD calculations on one specific exemplary perforatedfin geometry. To keep the computation size and time reasonable, theauthors only included a minimum repeating structure as illustrated inFIGS. 2a and 2b on page 2 of the paper. The cross section modeled forthe perforated fin represents one half of a wavelength of a fin, whichincludes a half each of the top and bottom fin lengths and one full finheight. These in turn include a series of half perforations on the topand bottom and a series of full perforations on the fin height all alongthe flow length. The full structure, also as illustrated in FIG. 1D,corresponds to exactly one row of perforations along the top, bottom,and side of each fin channel along the flow length, all of which arelaterally aligned. The diameter of the perforations is 0.8 mm asillustrated in Table 1 and the spacing of the perforations along thefins appears to be approximately 1.4 mm from center to center as can beinferred from FIGS. 6C and 7C. This frequency of perforations representsapproximately 16% open area on only the sides of the plate fin passages(i.e., the Zhu and Li paper does not count or consider the perforationson the top or the bottom of the fins for determining open area becausefins perforations on the top and bottom of the fins are covered by theparting sheets). This open area determination is illustrated in Table 1under the column on specifications. Such a pattern would work out toapproximately 20% open area on the flat perforated sheet prior to itsbeing formed into fins. It appears that this geometry represents atypical case the authors chose to model with no indication or teachingas to what they might consider preferred in terms of perforationpatterns and geometry.

Thus, the one specific exemplary perforated fin geometry described aboveis merely a representative perforated fin that the authors used tocompare against the four types of fins (plain, perforated, strip offsetand wavy types). The pattern and geometry the authors modeled aredifferent from those taught under the current application.

In summary, prior descriptions concerning perforated fins were brief inthe details concerning the geometry of the perforated fins used in platefin exchangers. And even when aspects of the geometry such as open areawere cited, there is no teaching on how to position the perforations orhow to select the best geometry for the perforations to obtain the bestperformance so that the overall capital and operating costs of the platefin heat exchangers may be minimized.

It is desired to increase the efficiency and improve the performance ofplate-fin heat exchangers.

It is further desired to improve the turbulence characteristics of asingle phase stream within the plate-fin passages of a plate-finexchanger in order to improve the heat transfer efficiency.

It is still further desired to have a plate-fin exchanger that exhibitshigh performance characteristics for cryogenic applications, such asthose used in air separation, and for other heat transfer applications.

It is still further desired to have a more efficient air separationprocess utilizing a plate-fin exchanger which is more compact and/ormore efficient than previously disclosed.

It is still further desired to have a plate-fin exchanger design whichminimizes the size, weight, and/or cost of the heat exchangers, whichwould result in an air separation process more efficient and/or lessexpensive per unit quantity of product produced.

It also is further desired to have a method for assembling a plate-finheat exchanger which uses fins with perforation patterns and geometrythat affords better performance than the fins previously disclosed, andwhich overcomes the disadvantages of the fins previously disclosed toprovide better and more advantageous results.

SUMMARY

The disclosed embodiments satisfy the need in the art by providing novelpatterns and novel geometry of fin perforations for use in plate finheat exchangers to maximize the overall heat transfer performance withinthe allowable pressure drop constraints. The benefits of such novelpatterns and novel geometry of fin perforations over previouslydisclosed fin patterns and geometry include: (1) a significant reductionin the volume; (2) a significant increase in heat transfer efficiency;(3) a significant reduction in pressure drop losses; or (4) somejudicious combination of factors (1) to (3) such that the overallcapital and operating cost of the heat exchanger system is reduced,thereby also reducing the capital and operating cost of the process thatutilizes such a heat exchanger system.

While the disclosed embodiments contained herein are mainly aimed ateasyway fins, wherein the flow is largely parallel to the fin flowchannels, the teachings may also be applicable to distribution fins,which simultaneously perform some heat transfer function and wherein theflow is predominantly, but not exclusively, parallel to the fin flowchannels. The embodiments disclosed herein are particularly suitable forapplications in which the fluid streams experience heat transfer withoutany phase change over at least 80% of the flow length, more preferablyover at least 90% of the flow length, and most preferably over 100% ofthe flow length within the plate-fin passages of the plate finexchanger, for example, containing fin channels with the perforationpatterns and geometry disclosed herein.

In a first embodiment a plate fin heat exchanger is disclosed comprisinga folded fin sheet comprising fins having a height, a width, and alength, the folded fin sheet being positioned between a first partingsheet and a second parting sheet; and a first side bar and a second sidebar, wherein the first side bar is positioned between the first partingsheet and the second parting sheet and adjacent to a first side of thefolded fin sheet, and wherein the second side bar is positioned betweenthe first parting sheet and the second parting and adjacent to a secondside of the folded fin sheet thereby forming at least a part of a platefin passage; wherein the fin sheet comprises a plurality ofperforations, such plurality of perforations are positioned on the finsheet in parallel rows when such fin sheet is in an unfolded state, suchparallel rows of perforations on the fin sheet comprise a first spacingbetween the parallel rows of perforations (S1), a second spacing betweensequential perforations within the parallel row of perforations (S2), athird spacing (or offset) between the perforations in adjacent parallelrows of perforations (S3), and a perforation diameter (D), wherein theratio of the first spacing between the parallel rows of perforations tothe perforation diameter (S1/D) is in the range of 0.75 to 2.0, andwherein the angle between the fins and the parallel rows of perforationsis less than or equal to five degrees (≦5°).

In a second embodiment a process for exchanging heat between at leasttwo streams in a plate fin heat exchanger in accordance with the firstembodiment is disclosed, wherein at least one stream undergoes heattransfer without phase change over at least 80% of the length of theplate-fin passages, and wherein the Reynolds Number of the at least onestream is in the range of 800 to 100,000 and more preferably in therange of 1,000 to 10,000.

In a third embodiment, a process for separating nitrogen, oxygen and/orargon from air by cryogenic distillation, which utilizes the plate finheat exchanger in accordance with the first embodiment is disclosed,wherein at least one stream undergoes heat transfer without phase changeover at least 80% of the length of the plate-fin passages, morepreferably over at least 90% of the length of the plate-fin passages,and most preferably over 100% of the length of plate-fin passages.

In a fourth embodiment, a method for manufacturing a plate fin heatexchanger is disclosed, which comprises the steps of: providing at leastone perforated sheet, the at least one perforated sheet comprising aplurality of perforations arranged in parallel rows, wherein suchparallel rows of perforations on the perforated sheet comprise a firstspacing between the parallel rows of perforations (S1), a second spacingbetween sequential perforations within the parallel row of perforations(S2), a third spacing (or offset) between the perforations in adjacentparallel rows of perforations (S3), and a perforation diameter (D),wherein the ratio of the first spacing between the parallel rows ofperforations to the perforation diameter (S1/D) is in the range of 0.75to 2.0; folding the at least one perforated sheet into fins to form afolded perforated sheet such that the angle between the fins and theparallel rows of perforations is less than or equal to five degrees(≦5°); positioning a first side bar adjacent to a first side of the atleast one folded perforated sheet, a second side bar adjacent to asecond side of the at least one folded perforated sheet, a firstdistributor fin adjacent to a first end of the at least one foldedperforated sheet, a second distributor fin adjacent to a second end ofthe at least one folded perforated sheet, a first endbar adjacent to thefirst distributor fin, and a second endbar adjacent to the seconddistributor fin to form a preliminary plate fin passage; placing thepreliminary plate fin passage of step (c) between a first parting sheetand a second parting sheet thereby forming a plate fin passagetherebetween; combining the plate fin passage of step (d) with otherplate fin passages to form the plate fin heat exchanger; and brazing theplate fin heat exchanger.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofexemplary embodiments, is better understood when read in conjunctionwith the appended drawings. For the purpose of illustrating embodiments,there is shown in the drawings exemplary constructions; however, theinvention is not limited to the specific methods and instrumentalitiesdisclosed. In the drawings:

FIG. 1 is an exploded perspective view of a basic element orsub-assembly of a plate-fin heat exchanger with fins having aperforation pattern and geometry according to one embodiment of thepresent invention;

FIG. 2 is a schematic diagram illustrating an embodiment of theperforation pattern on a flattened plate prior to their being formedinto fins according to the present invention; and

FIG. 3 is a graph illustrating the relative heat transfer and pressureloss performance of perforated fins as a function of S1/D with anindication of the preferred range.

DETAILED DESCRIPTION

One embodiment of the current invention relates to plate fin exchangersthat comprise perforated fins in at least a portion of the plate-finpassages and to the methods for assembling such plate fin exchangers.The perforated fins are assembled using flat perforated sheets. Theformed fins have a special relationship to the perforation pattern onthe flat sheet. While some plate-fin passages have the aforesaid fins,other plate-fin passages may have different types of fins, includingplain, perforated, strip offset and wavy types, for example. Plate-finheat exchangers that comprise such perforated fins have particularapplication in cryogenic processes such as air separation, although theymay also be used in other heat transfer processes.

Referring to FIG. 1, a plate-fin heat exchanger of the current inventioncomprises several plate fin passages, some of which are made by placingat least one fin sheet 10 in between parting sheets or plates 30,40sidebars 50,60, distributing fins (not shown but generally known in theart) and endbars (not shown but generally known in the art). Theseplate-fin passages comprise special patterns of perforations 20 in atleast some portion of such plate-fin passages.

Prior to being formed into the fin sheet 10 as illustrated in FIG. 1,the fin sheet 10 is a flattened sheet made of a metal such as aluminum,copper, another alloy, or any other heat conducting material known inthe art for fabrication of fins. The flattened fin sheet 10, asillustrated in FIG. 2, comprises the perforations 20. The flattenedsheet has special perforation patterns comprising several parallel rowsof perforations 100,200,300 with each parallel row 100,200,300comprising perforations 1A,1B,1C; 2A,2B,2C; and 3A,3B,3C. In oneembodiment, the rows of perforations 1A,1B,1C; 2A,2B,2C; and 3A,3B,3Cwill align in a direction that is parallel to the desired direction ofthe fins when the flattened sheet is folded to form the fin sheet 10 asdepicted in the FIG. 1. When the fins are employed as easyway fins, thenominal stream lines of the flow will be parallel to the direction ofperforations as illustrated in FIG. 2.

As illustrated in FIG. 2, the perforations have a diameter (D). Thespacing between parallel rows of perforations 100, 200, 300 isdesignated S1 while the spacing between sequential perforations (i.e.,between perforation 2A and 2B) in the stream flow direction isdesignated as S2. The offset between perforations in adjacent parallelrows 100, 200, 300 (i.e. between 2A and 3A) is designated as S3.

In one embodiment, Applicants found with surprising result that when thefollowing parameters are held within the following ranges: (1)perforation diameters D in the range of 1 mm to 4 mm; (2) open area inthe range of 5% to 25%; (3) the ratio S3/S2 in the range of 0.25-0.75;(4) and the ratio S1/D in the range of 0.75 to 2.0 with a most preferredrange of 0.75 to 1.0, the plate-fin heat exchangers exhibited higherefficiencies and improved the performance compared with traditional heatexchangers not designed accordingly.

In the most preferred arrangement/embodiment, the fluid flow directionis parallel to the parallel rows of perforations 100,200,300, but in apreferred arrangement/embodiment the direction of fluid flow is withinfive degrees (5%) to the direction of the parallel rows of perforations100,200,300. This means that as the fins are formed, the fin sheet 10should be folded such that the angle between the fin folds and suchparallel rows of perforations 100,200,300 is less than or equal to fivedegrees, while the most preferred arrangement is where such angle iszero degrees (0°).

The fin sheets 10 may comprise perforations 20 that are circular asillustrated in FIGS. 1 and 2, however, those skilled in the art willrecognize that non-circular perforations may also be used including, butnot limited to perforations in the shape of ellipses, rectangles,parallelograms, and other such shapes.

In yet another embodiment, the arrangement of the offset rows ofperforations will repeat every two rows as illustrated in FIG. 2 (i.e.,Row 100 shall be offset similar to Row 300, 500 (not shown), 700 (notshown), etc.). Further, when the flat perforated sheets are folded intofins in a finning operation, the structure of perforations that resulton the finished fin sheet 10 tend to have a complex relationship becauseof the mechanical details of how the material flows in the finning dies.In one embodiment, the flattened sheet is folded such that theperforation patterns on the finished fin sheet 10 repeat at least onceevery ten (10) fin wavelengths and more preferably at least once everyfive (5) fin wavelengths, in at least fifty percent (50%) of the heatexchanger plate-fin passages containing such perforated fins, morepreferably in at least eighty percent (80%) of the plate-fin passagesand most preferably in one-hundred percent (100%) of the plate-finpassages.

In a further embodiment, surface texture may be applied to theperforated sheets prior to the material being folded into fins as taughtby U.S. Pat. No. 6,834,515 B2, entitled “Plate Fin Exchangers withTextured Surfaces,” to Sunder et al., that is incorporated by referencein its entirety. Alternatively the surface texture may be created in theprocess of creating the fins from the flat perforated sheets.

The embodiments described herein are suitable for plate fin heatexchangers wherein at least a portion of the fins have a height in therange of 0.25 inches to 1 inch (0.635 centimeters to 2.54 centimeters),more preferably in the range of 0.40 inches to 0.75 inches (1.016centimeters to 1.905 centimeters) and most preferably in the range of0.5 inches to 0.6 inches (1.27 centimeters to 1.524 centimeters). Theembodiments are advantageously applied when the fluid flow conditions insuch plate fin passages are in a transition state between laminar andturbulent states or in a turbulent state. This may be expressed as aReynolds Number range of 800 to 100,000 and more preferably a range of1,000 to 10,000. The Reynolds Number is calculated as follows:

Re=ρVD/μ, where

Where,

Re=Reynolds number;

ρ=fluid density;

V=fluid velocity;

μ=fluid viscosity;

D=4A/P;

A=fluid flow cross sectional area; and

P=fluid flow perimeter

For plate-fin passages, it is common to calculate the hydraulic diameterD based on individual plate-fin passages and the current calculationsare based on using the base metal sheets without adjusting for theperforations for their contributions to the A (fluid flow crosssectional area) and P (fluid flow perimeter).

Embodiments of the present invention have significant value becauseplate fin heat exchangers may be made more compact relative toconventional plate-fin exchangers, thus, saving combined capital andoperating costs of the plant, such as an air separation plant.

Example 1

To better understand the influence of the perforations within the fingeometry, several sample problems were solved using Computational FluidDynamics (CFD). In using this technique, it is common to restrict thecomputation to some repeating structure in order to limit thecomputational size of the problem. But when one tries to quantify theeffect of specific perforation patterns, the overall geometry of theheat exchanger is very complicated, even when one limits the problem toa single subchannel within plate-fin passages. For this reason adifferent type of approximation was used.

In most plate fin exchangers the secondary surface area tends to be thedominant fraction of the total area. As noted before, this is the arearepresented by the fin legs that span and separate the parting sheets orplates 30,40 that represent the primary surface area. To understand theeffect of the positioning of the perforations, a representative periodicarea of two infinite parallel plates was modeled to quantify the heattransfer and pressure losses that occur when air flows between them. Thegeneral scheme of the perforations on the flattened sheet is illustratedin FIG. 2.

Example 1 concerns easyway fins that are used for heat transfer and/ordistribution purposes, wherein, as previously stated, the direction offlow is generally parallel to the fin direction as indicated in FIG. 2.

A number of exemplary cases were solved using CFD, wherein the variousspacings (S1, S2, and S3) were varied while keeping the diameter (D) ofthe perforations and the overall open area constant. Specifically,spacings S1 and S2 were varied simultaneously, while the offset S3 wasset equal to one-half of the spacing S2. In these exemplary cases, therewas only one independent parameter and the results are listed in Table 1and illustrated in FIG. 3.

TABLE 1 Relative pressure Parameter S1/S2 S1/D Relative heat transferloss Case 1 0.037 0.5417 1.2626 1.2140 Case 2 0.071 0.7500 1.2469 1.1806Case 3 0.127 1.0000 1.2465 1.1789 Case 4 0.224 1.3292 1.2162 1.1689 Case5 0.348 1.6583 1.1951 1.1554 Case 6 0.500 1.9875 1.1881 1.1505 Case 70.679 2.3167 1.1347 1.1031 Case 8 0.886 2.6458 1.0632 1.0483 Case 91.120 2.9750 1.0000 1.0000

The exemplary calculations show the relative values of the pressurelosses and heat transfer rates that are obtained merely by changing thepattern of perforations. The exemplary data was plotted after scalingrelative to the values that occur when the ratio of spacing to theperforation dimension was approximately 3. As this ratio is lowered toapproximately 2, significant improvement occurs in heat transfer. Asnoted in Table 1, the increase in heat transfer is higher than theincrease in the corresponding pressure loss. Thus, a heat exchangerdesigned at a ratio of 2 may be shorter by a factor of about 1.2compared to a heat exchanger designed at a ratio of 3, while the overallpressure loss will also be lower. This is a significant reduction inlength and thereby the volume. If the ratio is reduced below 2, theimprovement continues and particularly good values are obtained betweenthe values of the ratio between 0.75 and 1. In this range of ratiosthere is an improvement in heat transfer by a factor of about 1.25. Thelength or volume required will be the reciprocal of this ratio namely0.80 or eighty percent (80%). This represents a substantial sizereduction by twenty percent (20%) while the pressure loss will also bereduced by the ratio of 1.18/1.25 which is equal to 0.94 or ninety-fourpercent (94%). Thus, there can be a twenty percent (20%) reduction inlength or volume while there is also a six percent (6%) reduction inpressure loss.

These are significant improvements that can be obtained by arranging theperforation positions as disclosed herein which was not known ordisclosed previously. In fact, either through express statements,implication, or illustrations, some previous disclosures taught awayfrom such arrangements. As illustrated in FIG. 3, the range of ratiosfrom 0.75 to 2.0 is preferred wherein the range from 0.75 to 1.0 isparticularly preferred.

Example 2

Example 2 illustrates an exemplary improvement obtained using theteaching contained herein. As noted before, traditional teachingsconcerning perforated fins in plate fin heat exchangers did not discusspreferred geometry or perforation patterns as outlined herein. The CFDpaper by Zhu et al., cited earlier, however, did study the effect of aspecific perforated fin in comparison with other forms of fins such asplain, serrated and wavy fins. The current example has been generated byapplying the perforation pattern used in the CFD paper by Zhu et al. inthe same manner as described in Example 1.

The parameters of the perforation pattern on the flat sheets beforebeing folded into fins are as follows: perforation diameter (D)=0.8 mm;open area=20%; S1=1.81 mm; S2=1.39 mm; and S3=0. The calculated relativeperformance of a heat exchanger that utilizes such prior art fins isshown in Table 2.

TABLE 2 Disclosed exemplary Parameter CFD Paper embodiment Perforationdiameter, mm 0.8 2.4 Open area, % 20 10 S1, mm 1.81 2.4 S2, mm 1.3918.96 S3, mm 0.0 9.48 S1/D 2.26 1.0 S3/S2 0.0 0.5 Relative heat transfercoefficient 1.00 1.26 Relative pressure gradient 1.00 1.26 Relativelength of exchanger 1.00 0.79 Relative volume of exchanger 1.00 0.79Relative pressure loss in 1.00 1.00 exchanger

As illustrated in Table 2, because the Relative heat transfercoefficient and Relative pressure gradient of the disclosed exemplaryembodiment are 26% higher than the CFD paper heat exchanger, a heatexchanger constructed according to the teachings of the disclosedexemplary embodiment can have a lesser relative length (21% less) and alesser relative volume (21% less) compared with a heat exchangerconstructed based on the teachings of the CFD paper where both heatexchangers have equal or matching heat transfer duty and pressure drop.This is a substantial benefit for utilizing fins made in accordance withthe teachings of the disclosed exemplary embodiment over the teachingsof the CFD paper.

While aspects of the present invention has been described in connectionwith the preferred embodiments of the various figures, it is to beunderstood that other similar embodiments may be used or modificationsand additions may be made to the described embodiment for performing thesame function of the present invention without deviating therefrom. Forexample, the following aspects should also be understood to be a part ofthis disclosure:

Aspect 1. A plate fin heat exchanger, comprising:

a folded fin sheet comprising fins having a height, a width, and alength, the folded fin sheet being positioned between a first partingsheet and a second parting sheet; and

a first side bar and a second side bar, wherein the first side bar ispositioned between the first parting sheet and the second parting sheetand adjacent to a first side of the folded fin sheet, and wherein thesecond side bar is positioned between the first parting sheet and thesecond parting and adjacent to a second side of the folded fin sheetthereby forming at least a part of a plate fin passage;

wherein the fin sheet comprises a plurality of perforations, suchplurality of perforations are positioned on the fin sheet in parallelrows when such fin sheet is in an unfolded state, such parallel rows ofperforations on the fin sheet comprise a first spacing between theparallel rows of perforations (S1), a second spacing between sequentialperforations within the parallel row of perforations (S2), a thirdspacing (or offset) between the perforations in adjacent parallel rowsof perforations (S3), and a perforation diameter (D), wherein the ratioof the first spacing between the parallel rows of perforations to theperforation diameter (S1/D) is in the range of 0.75 to 2.0, and whereinthe angle between the fins and the parallel rows of perforations is lessthan or equal to five degrees (≦5°).

Aspect 2. The plate fin heat exchanger of Aspect 1, wherein the anglebetween the fins and the parallel rows of perforations is zero degrees(0°).

Aspect 3. The plate fin heat exchanger of Aspect 1 or Aspect 2, whereinthe ratio of the first spacing between the parallel rows of perforationsto the perforation diameter (S1/D) is in the range of 0.75 to 1.0.

Aspect 4. The plate fin heat exchanger of any one of Aspects 1 to Aspect3, wherein the ratio of the third spacing (or offset) betweenperforations in adjacent parallel rows of perforations (S3) and thesecond spacing between sequential perforations within the parallel rowof perforations (S2) is in the range of 0.25 to 0.75.

Aspect 5. The plate fin heat exchanger of any one of Aspects 1 to Aspect4, wherein 5% to 25% of the area of the folded fin sheet in the unfoldedstate is occupied by the perforations.

Aspect 6. The plate fin heat exchanger of any one of Aspects 1 to Aspect5, wherein the perforation diameter (D) is in the range of 1 mm to 4 mm.

Aspect 7. The plate fin heat exchanger of any one of Aspects 1 to Aspect6, wherein the perforations are circular.

Aspect 8. The plate fin heat exchanger of any one of Aspects 1 to Aspect6, wherein the perforations are in the shape of ellipses, rectangles, orparallelograms.

Aspect 9. The plate fin heat exchanger of any one of Aspects 1 to Aspect8, wherein the adjacent parallel rows of perforations are offset inalternating fashion such that the position of the parallel rows ofperforations repeats every other row of perforations.

Aspect 10. The plate fin heat exchanger of any one of Aspects 1 toAspect 8, wherein the adjacent parallel rows of perforations are offsetsuch that the position of the parallel rows of perforations on the finsof the folded fin sheet repeat exactly at least once every 10 finwavelengths and more preferably at least once every 5 fin wavelengths,in at least 50% of the heat exchanger plate fin passages containing suchperforated fins, more preferably in at least 80% of the plate finpassages and most preferably in 100% of the plate fin passages.

Aspect 11. The plate fin heat exchanger of any one of Aspects 1 toAspect 10, wherein the folded fin sheet comprises a surface texture.

Aspect 12. The plate fin heat exchanger of any one of Aspects 1 toAspect 11, wherein the fin height is in the range of 0.25 inches to 1inch, more preferably in the range of 0.4 inches to 0.75 inches, andmost preferably in the range of 0.5 inches to 0.6 inches.

Aspect 13. The plate fin heat exchanger of any one of Aspects 1 toAspect 12, wherein the folded fin sheet is an easyway heat transfer finor distributor fin.

Aspect 14. The plate fin heat exchanger of any one of Aspects 1 toAspect 13, wherein the plate-fin passages are adapted to accept a fluidstream, and wherein the fluid stream undergoes heat transfer withoutphase change over at least 80%, more preferably over at least 90%, andmost preferably over 100% of the length of the plate-fin passages.

Aspect 15. A process for exchanging heat between at least two streams ina plate fin heat exchanger constructed in accordance with any one ofAspects 1 to Aspect 13, wherein at least one stream undergoes heattransfer without phase change over at least 80% of the length of theplate-fin passages, and wherein the Reynolds Number of the at least onestream is in the range of 800 to 100,000 and more preferably in therange of 1,000 to 10,000.

Aspect 16. A process for separating nitrogen, oxygen and/or argon fromair by cryogenic distillation, which utilizes the plate fin heatexchanger of any one of Aspects 1 to Aspect 13, wherein at least onestream undergoes heat transfer without phase change over at least 80% ofthe length of the plate-fin passages, more preferably over at least 90%of the length of the plate-fin passages, and most preferably over 100%of the length of plate-fin passages.

Aspect 17. A method for manufacturing a plate fin heat exchanger whichcomprises the steps of:

-   -   (a) providing at least one perforated sheet, the at least one        perforated sheet comprising a plurality of perforations arranged        in parallel rows, wherein such parallel rows of perforations on        the perforated sheet comprise a first spacing between the        parallel rows of perforations (S1), a second spacing between        sequential perforations within the parallel row of perforations        (S2), a third spacing (or offset) between the perforations in        adjacent parallel rows of perforations (S3), and a perforation        diameter (D), wherein the ratio of the first spacing between the        parallel rows of perforations to the perforation diameter (S1/D)        is in the range of 0.75 to 2.0;    -   (b) folding the at least one perforated sheet into fins to form        a folded perforated sheet such that the angle between the fins        and the parallel rows of perforations is less than or equal to        five degrees (≦5°);    -   (c) positioning a first side bar adjacent to a first side of the        at least one folded perforated sheet, a second side bar adjacent        to a second side of the at least one folded perforated sheet, a        first distributor fin adjacent to a first end of the at least        one folded perforated sheet, a second distributor fin adjacent        to a second end of the at least one folded perforated sheet, a        first endbar adjacent to the first distributor fin, and a second        endbar adjacent to the second distributor fin to form a        preliminary plate fin passage;    -   (d) placing the preliminary plate fin passage of step (c)        between a first parting sheet and a second parting sheet thereby        forming a plate fin passage therebetween;    -   (e) combining the plate fin passage of step (d) with other plate        fin passages to form the plate fin heat exchanger; and    -   (f) brazing the plate fin heat exchanger.

Aspect 18. A method for manufacturing a plate fin heat exchangeraccording to Aspect 17, further comprising applying a surface texture toat least one perforated sheet prior to folding the at least oneperforated sheet in step (b).

The claimed invention, therefore, should not be limited to any singleembodiment or aspect, but rather should be construed in breadth andscope in accordance with the appended claims.

1. A plate fin heat exchanger, comprising: a folded fin sheet comprisingfins having a height, a width, and a length, the folded fin sheet beingpositioned between a first parting sheet and a second parting sheet; anda first side bar and a second side bar, wherein the first side bar ispositioned between the first parting sheet and the second parting sheetand adjacent to a first side of the folded fin sheet, and wherein thesecond side bar is positioned between the first parting sheet and thesecond parting and adjacent to a second side of the folded fin sheetthereby forming at least a part of a plate fin passage; wherein the finsheet comprises a plurality of perforations, such plurality ofperforations are positioned on the fin sheet in parallel rows when suchfin sheet is in an unfolded state, such parallel rows of perforations onthe fin sheet comprise a first spacing between the parallel rows ofperforations (S1), a second spacing between sequential perforationswithin the parallel row of perforations (S2), a third spacing (oroffset) between the perforations in adjacent parallel rows ofperforations (S3), and a perforation diameter (D), wherein the ratio ofthe first spacing between the parallel rows of perforations to theperforation diameter (S1/D) is in the range of 0.75 to 2.0, and whereinthe angle between the fins and the parallel rows of perforations is lessthan or equal to five degrees (≦5°).
 2. The plate fin heat exchanger ofclaim 1, wherein the angle between the fins and the parallel rows ofperforations is zero degrees (0°).
 3. The plate fin heat exchanger ofclaim 1, wherein the ratio of the first spacing between the parallelrows of perforations to the perforation diameter (S1/D) is in the rangeof 0.75 to 1.0.
 4. The plate fin heat exchanger of claim 1, wherein theratio of the third spacing (or offset) between perforations in adjacentparallel rows of perforations (S3) and the second spacing betweensequential perforations within the parallel row of perforations (S2) isin the range of 0.25 to 0.75.
 5. The plate fin heat exchanger of claim1, wherein 5% to 25% of the area of the folded fin sheet in the unfoldedstate is occupied by the perforations.
 6. The plate fin heat exchangerof claim 1, wherein the perforation diameter (D) is in the range of 1 mmto 4 mm.
 7. The plate fin heat exchanger of claim 1, wherein theperforations are circular.
 8. The plate fin heat exchanger of claim 1,wherein the perforations are in the shape of ellipses, rectangles, orparallelograms.
 9. The plate fin heat exchanger of claim 1, wherein theadjacent parallel rows of perforations are offset in alternating fashionsuch that the position of the parallel rows of perforations repeatsevery other row of perforations.
 10. The plate fin heat exchanger ofclaim 1, wherein the adjacent parallel rows of perforations are offsetsuch that the position of the parallel rows of perforations on the finsof the folded fin sheet repeat exactly at least once every 10 finwavelengths and more preferably at least once every 5 fin wavelengths,in at least 50% of the heat exchanger plate fin passages containing suchperforated fins, more preferably in at least 80% of the plate finpassages and most preferably in 100% of the plate fin passages.
 11. Theplate fin heat exchanger of claim 1, wherein the folded fin sheetcomprises a surface texture.
 12. The plate fin heat exchanger of claim1, wherein the fin height is in the range of 0.25 inches to 1 inch, morepreferably in the range of 0.4 inches to 0.75 inches, and mostpreferably in the range of 0.5 inches to 0.6 inches.
 13. The plate finheat exchanger of claim 1, wherein the folded fin sheet is an easywayheat transfer fin or distributor fin.
 14. The plate fin heat exchangerof claim 1, wherein the plate-fin passages are adapted to accept a fluidstream, and wherein the fluid stream undergoes heat transfer withoutphase change over at least 80%, more preferably over at least 90%, andmost preferably over 100% of the length of the plate-fin passages.
 15. Aprocess for exchanging heat between at least two streams in a plate finheat exchanger constructed in accordance with claim 1, wherein at leastone stream undergoes heat transfer without phase change over at least80% of the length of the plate-fin passages, and wherein the ReynoldsNumber of the at least one stream is in the range of 800 to 100,000 andmore preferably in the range of 1,000 to 10,000.
 16. A process forseparating nitrogen, oxygen and/or argon from air by cryogenicdistillation, which utilizes the plate fin heat exchanger of claim 1,wherein at least one stream undergoes heat transfer without phase changeover at least 80% of the length of the plate-fin passages, morepreferably over at least 90% of the length of the plate-fin passages,and most preferably over 100% of the length of plate-fin passages.
 17. Amethod for manufacturing a plate fin heat exchanger which comprises thesteps of: (a) providing at least one perforated sheet, the at least oneperforated sheet comprising a plurality of perforations arranged inparallel rows, wherein such parallel rows of perforations on theperforated sheet comprise a first spacing between the parallel rows ofperforations (S1), a second spacing between sequential perforationswithin the parallel row of perforations (S2), a third spacing (oroffset) between the perforations in adjacent parallel rows ofperforations (S3), and a perforation diameter (D), wherein the ratio ofthe first spacing between the parallel rows of perforations to theperforation diameter (S1/D) is in the range of 0.75 to 2.0; (b) foldingthe at least one perforated sheet into fins to form a folded perforatedsheet such that the angle between the fins and the parallel rows ofperforations is less than or equal to five degrees (≦5°); (c)positioning a first side bar adjacent to a first side of the at leastone folded perforated sheet, a second side bar adjacent to a second sideof the at least one folded perforated sheet, a first distributor finadjacent to a first end of the at least one folded perforated sheet, asecond distributor fin adjacent to a second end of the at least onefolded perforated sheet, a first endbar adjacent to the firstdistributor fin, and a second endbar adjacent to the second distributorfin to form a preliminary plate fin passage; (d) placing the preliminaryplate fin passage of step (c) between a first parting sheet and a secondparting sheet thereby forming a plate fin passage therebetween; (e)combining the plate fin passage of step (d) with other plate finpassages to form the plate fin heat exchanger; and (f) brazing the platefin heat exchanger.
 18. A method for manufacturing a plate fin heatexchanger according to claim 17, further comprising applying a surfacetexture to at least one perforated sheet prior to folding the at leastone perforated sheet in step (b).